Cryorefrigeration Device and Method of Implementation

ABSTRACT

The invention proposes a reliable high-performance cryorefrigeration device, the level of thermal oscillations of which may be minimized so as to be below a threshold value. For this purpose, the subject of the invention is a cryorefrigeration device comprising N periodically operating cryorefrigerators ( 100 ), N being an integer equal to or greater than 2, each cryorefrigerator being provided with a cold end coupled to a common cold end ( 300 ), the cryorefrigerators being linked to a control device provided with a phase-shift means capable of making the cryorefrigerators operate in phase-shifted relationship one with respect to another.

The present invention relates to a cryorefrigeration device and a methodof use thereof.

A cryorefrigeration device is a cyclically operating apparatus producingcooling capacity at a temperature below 120 K, without material removaloutside the cycle.

Among known cryorefrigerators, pulse tube refrigerators are particularlyadvantageous, owing to the absence of moving parts at low temperature,which results in a low level of vibrations generated high reliabilityand long service life. Pulse tube refrigerators are thereforeadvantageously used in the space field or for cooling sensitivedetectors.

The operation of cryorefrigerators (pulse tube refrigerators,Gifford-MacMahon refrigerators, etc.) is based on a reciprocating cycleusing a gas (advantageously helium). The temperature behavior of acryorefrigerator is not perfectly sinusoidal, but it is periodic. Thecooling capacity is obtained by successive expansions of the cyclehelium, cadenced by a rotary valve.

Cryorefrigerators comprise a drive portion (compression) whichcirculates the gas in a second “cold head” portion, incompression/expansion cycles, to generate a useful temperature for theuser.

The cold head has a generally elongated shape of which the free end(that is to say opposite the drive portion) represents the usefulinterface for the user, and is called the “cold end”. The cold end isenclosed in an insulating chamber which contains the object to becooled. The interior of the chamber is placed under vacuum to limitentries of heat.

The reciprocating operation naturally generates high thermaloscillations at the cold end of the cryorefrigerator. This cold end,generally made from copper, has a much higher mass than the cycle gasundergoing the compressions-expansions. It is, however, incapable ofeffectively smoothing the oscillations by thermal inertia, because atthe temperatures concerned (typically 3 to 20 K), solid materials have avery low volumetric heat capacity compared to that of helium.

Thermal oscillation measurements were taken on various cryorefrigerators(Gifford-MacMahon and pulse tube refrigerators). The peak-to-peakamplitudes temperature taken at the cold end were measured between 300mK and 2200 mK. These thermal oscillations are therefore considerableand are disturbing for certain applications.

Systems intended to reduce the mechanical vibrations ofcryorefrigerators are proposed by manufacturers: a “sock” surrounds thecold end, with a clearance filled with heat exchange gas (helium). Thissystem procures a (moderate) damping of thermal oscillations and henceof the mechanical vibrations, but at the cost of a wide temperaturedifference between the useful temperature for the user and the cold endtemperature, which is detrimental to performance. In other words, thetemperature usable by the user will not be as low as the temperaturegenerated by the pulse tube refrigerator.

Similarly, it has been proposed to reduce the thermal oscillations byfiltering, by associating a thermal resistance and a heat capacity witha pulse tube refrigerator, as for an RC circuit in electricity. However,while the presence of an artificially introduced thermal resistance iseffective for filtering, it has the drawback of also generating a widetemperature difference between the useful temperature for the user andthe cold end temperature.

Document US 2005/0028534 describes a cryorefrigeration deviceincorporating a plurality of periodically operating cryorefrigerators.To reduce the level of vibrations generated at the cold end by thecyclic operation of the cryorefrigerators, this document proposes aphase-shifted operation of the cryorefrigerators on the one hand, andthe merging of the cold ends of the individual cryorefrigerators into asingle common cold end, on the other hand. This particular arrangementconsists in connecting the tubes of each cryorefrigerator to the commoncold end in order to obtain a symmetrical arrangement about the centerof the common cold end. Thus, during the gas compression/expansioncycles, the deformations on the tubes are partially offset and produce alower level of vibrations at the common cold end than for individualcryorefrigerators.

However, such a solution necessarily implies the fabrication of aspecially designed multiple cryorefrigerator, in which the cold endconnects the various tubes of the individual cryorefrigerators.

It is the object of the invention to overcome these drawbacks byproposing an economical cryorefrigeration device, that is to say, whichcan be made with commercial cryorefrigerators, which is reliable,efficient, that is to say, without a significant difference between thetemperature generated and the useful temperature, and in which the levelof thermal oscillations may be reduced below a threshold value,preferably less than 2% of the mean temperature generated.

For this purpose, the invention relates to a cryorefrigeration devicecomprising N periodically operating cryorefrigerators, where N is aninteger equal to or greater than 2, each provided with a cold endconnected to a common cold end, the cryorefrigerators being associatedwith a control device provided with a phase-shift means suitable foractuating phase-shifted operation of the cryorefrigerators with regardto one another, the cold end of each cryorefrigerator being connected tothe common cold end via a heat conducting mechanical uncoupling means.

According to other features of the invention:

the phase-shift means may be suitable for actuating a phase-shiftedoperation with a phase difference of 2π/N, to within 5 degrees, betweeneach cryorefrigerator;

the cryorefrigerators may be identical;

the cryorefrigerators may be pulse tube refrigerators orGifford-MacMahon cryorefrigerators;

the heat conducting mechanical uncoupling means may comprise a pluralityof heat conducting wire braids fixed between two mounting plates inthermal contact with the cold end of a cryorefrigerator and the commoncold end, respectively;

the heat conducting wires of the braids may be made from Cu/a1 copperand have a diameter between 0.03 mm and 0.1 mm, preferably equal to 0.05mm, and the mounting plates have a residual resistivity ratio of atleast 50;

the mounting plates and/or the common cold end may be made from Cu/a1 orCu/c1 copper;

the heat conducting wires of the braids may be welded to the mountingplates by electron beam welding so as to ensure material continuity;and/or

the common cold end may comprise a temperature sensor connected to thecontrol device.

The invention also relates to a method for using the abovecryorefrigeration device, comprising a step of actuating the Ncryorefrigerators in a phase-shifted manner with regard to one another.

According to a first embodiment of the invention, the method comprises astep of adjusting the phase difference between each of the Ncryorefrigerators, this adjustment step comprising the following steps:

-   -   a1) operating the N cryorefrigerators simultaneously, in any        phase-shifted manner;    -   b1) measuring the temperature variations of the common cold end        with the temperature sensor and calculating the mean temperature        of the common cold end;    -   c1) actuating a phase-shifted operation between each        cryorefrigerator until the temperature variations of the common        cold end during the operation of the cryorefrigeration device        are lower than a threshold value, preferably less than 2% in        absolute value of the mean temperature of the common cold end,        advantageously less than 1% in absolute value of the mean        temperature of the common cold end; and/or    -   d1) setting the operating phase of each of the N        cryorefrigerators.

According to a second embodiment of the invention, the method comprisesa step of adjusting the phase difference between each of the Ncryorefrigerators, this adjustment step comprising the following steps:

-   -   a2) operating the N cryorefrigerators simultaneously, with a        phase difference of 2π/N between each cryorefrigerator;    -   b2) measuring the temperature variations of the common cold end        with the temperature sensor and calculating the mean temperature        of the common cold end;    -   c2) varying the phase of N-1 cryorefrigerators about their        initial phase in step a) until the temperature variations of the        common cold end during the operation of the cryorefrigeration        device are lower than a threshold value, preferably less than 2%        in absolute value of the mean temperature of the common cold        end, advantageously less than 1% in absolute value of the mean        temperature of the common cold end;    -   d2) setting the operating phase of each of the N        cryorefrigerators.

By convention, the steps of this method are carried out in alphabeticalorder.

Other features of the invention are described in the detaileddescription below with reference to the appended figures which show,respectively:

in FIG. 1, a schematic perspective view of a cryorefrigeration deviceaccording to the invention;

in FIG. 2, a schematic perspective view of a thermal coupler (commoncold end) provided with two mechanical uncoupling means according to theinvention, intended to be fixed in thermal contact with the cold ends ofthe two cryorefrigerators;

in FIG. 3, a graph showing the cold end temperature of two phase-shiftedcryorefrigerators and the temperature of the thermal coupler;

in FIG. 4, a diagram showing the phase-shift regulation of a system forimplementing the device according to the invention;

in FIG. 5, a graph showing the thermal oscillations of the common coldend when the phase difference between two pulse tube refrigerators isvaried continuously, for two mean temperature levels of the common coldend; and

in FIG. 6, a schematic plan view of a second embodiment of acryorefrigeration device according to the invention.

With reference to FIG. 1, a cryorefrigeration device according to theinvention comprises two identical periodically operatingcryorefrigerators 100. These cryorefrigerators are pulse tuberefrigerators of a known type (for example CRYOMECH PT415 pulse tuberefrigerators) the structure of which is shown in FIG. 1, but is notcompletely described in detail, as those skilled in the art know thisstructure.

Chiefly, each cryorefrigerator 100 comprises a drive portion 110 whichcirculates the gas in a second “cold head” portion 120, incompression/expansion cycles in order to generate a useful temperaturefor the user.

The drive portion 110 is connected, in the embodiment shown, to acompressor (not shown).

The cold head 120 has a generally elongated shape of which the endopposite the drive portion is the cold end 121.

The two cryorefrigerators 100 are mounted side by side on a commonmounting plate 200 of a vacuum chamber and supported mechanically by amounting flange 101.

During operation, the drive portion of the plate 200 supporting the twocryorefrigerators remains at ambient temperature (300 K). Thus, thecenterline distance between the two machines does not vary. On thecontrary, the cold head 120, located under the plate 200 is enclosed ina vacuum chamber and undergoes thermal contractions.

A coupling rod 300 of heat conducting material (preferably copper,advantageously Cu/a1 or Cu/C1 copper) is thermally connected to the coldends of the two cryorefrigerators, which it joins. This rod 300, or“thermal coupler”, constitutes the common cold end. The object to becooled is, as it would be mounted mounted on the coupling rod 300 underthe cold end in the case of a single pulse tube refrigerator.

The denominations of Cu a1 and Cu c1 according to standard NF A 51-050correspond to the following ISO 431 denominations:

-   -   Cu-a1 becomes Cu-ETP    -   Cu-c1 becomes Cu-OF

Cu-A1 (or Cu-ETP) copper has a copper content above 99.90%. This is anelectrolytically refined copper, not deoxidized, with guaranteedconductivity. Cu-C1 (or Cu-OF) copper has a copper content above 99.95%.This type of copper is an oxygen-free or deoxidized copper with traceresidual deoxidant. In any case, the residual deoxidant content is toolow to affect the conductivity.

The geometry of the coupling rod 300 is selected to optimize the thermalbehavior. For a number of pulse tubes higher than 2, the rodadvantageously has the shape of a triangle, a square, a disc, etc.

All of the coupling rod 300 is cooled to low temperature and contracts(4 mm/m for copper). For a cryorefrigeration device according to theinvention, with two pulse tubes, the rod may reach a length of aboutfifty centimeters, so that the contraction may reach about 2 mm. If thenumber of tubes increases, the length of the rod and hence thecontraction increase.

The invention provides for a heat conducting mechanical uncoupling means400 between the cryorefrigerators 100 and the coupling rod 300, so thatthe stress generated by the contraction of the rod is not transmitted tothe pulse tubes.

A first embodiment, not shown, consists in providing a bellows under themounting flanges of the cryorefrigerators at the plate of the vacuumchamber. The centerline distance of the cryorefrigerators is then freeto reduce by 1 to 2 mm during the contraction of the cold rod. However,very flexible bellows must be provided to limit the stresses generatedin the tubes of the refrigerators, while the external atmosphericpressure bears considerably on the bellows which communicate internallywith the vacuum of the chamber.

A second preferred embodiment is shown in FIGS. 1 and 2.

In this embodiment, the cold end 121 of each cryorefrigerator 100 isconnected to the common cold end 300 via a heat conducting mechanicaluncoupling means 400.

In an advantageous exemplary embodiment, the heat conducting mechanicaluncoupling means 400 comprises a plurality of braids 410 of heatconducting wires fixed between two mounting plates 420-430 in thermalcontact with the cold end 121 of a cryorefrigerator 100 and the commoncold end 300, respectively.

This exemplary embodiment must be designed to ensure the least possibledeterioration of the thermal connection between the common cold end andthe cold source consisting, here, of the pulse tubes. The heatconducting mechanical uncoupling means 400 must transmit a heat fluxwith the lowest possible temperature drop (ΔT).

Preferably, the heat conducting mechanical uncoupling means 400 is madeusing the shortest and largest possible number of copper braids. Thebraids are selected for their high thermal conductivity properties atlow temperature, and their high flexibility (very thin wires).

Preferably, the heat conducting wires of the braids are made from Cu/a1or Cu/c1 copper, and have a diameter between 0.03 mm and 0.1 mm,preferably 0.05 mm. The flexibility between the plates 420-430 and therod 300 depends considerably on the diameter of these wires.

The dimensions of the braids are the result of a mechanical-thermalcompromise. The heat transfer calculations encourage the use of braidshaving a high total cross section and short length (heat conduction) tolimit the temperature drop (ΔT) lost by conduction. The mechanicalcalculations encourage the use of the most flexible possible braids,hence the use of thin wires (typically having a diameter of 0.05 mm) andwhich are not too short (typically about 30 mm for braids with a crosssection of about 25 mm²). For example, a braid may consist of 12 strandsof 1062 wires having a diameter of 0.05 mm.

Also preferably, the mounting plates 420-430 have a residual resistivityratio (RRR) of at least 50. For this purpose, the mounting plates420-430 are selected to be made from copper having high thermalconductivity at low temperature (Cu/a1 or Cu/c1). The same applies tothe common cold end.

The residual resistivity ratio provides a good image of the lowtemperature conductivity of the copper. It can be measured according tointernational standard (IEC 61788-11) of 2003.

Alternatively, the following method can be used to calculate theresidual resistivity ratio of the mounting plates according to thepresent invention:

-   -   A slender test specimen having a cross section of about 4 mm²        and a length of about 150 mm is taken from the block of material        to be qualified.    -   The test specimen is mounted on a cryogenic rod intended to be        immersed in a Dewar flask of liquid helium.    -   The ends of two power input cables (diameter 1 mm) are soldered        with tin at the two ends of the test specimen.    -   Two other electrical cables (diameter 0.2 mm) serving as power        connectors are placed in contact (but not soldered) with the        ends of the test specimen, in the area not polluted by the tin        solder of the test specimen. This is because any tin that has        migrated into the copper can affect the value of the RRR. The        power connections are separated by a distance of about 100 mm.    -   With the rod and specimen at ambient temperature, a current of        10 A is sent through the specimen. The voltage generated between        the two power connections is measured using a microvoltmeter.    -   The rod is then immersed in the liquid helium, so that the        specimen is cooled completely to the helium saturation vapor        temperature (4.2 K).    -   With the 10 A current maintained across the specimen, the new        value of the potential difference developed along the specimen        is measured using the microvoltmeter.    -   The ratio of the voltage developed at ambient temperature to the        voltage at 4.2 K directly gives the RRR of the test specimen,        without the need to determine the real electrical resistance of        the specimen. RRR values above 50 are considered to be the most        effective for implementing the invention. Low-RRR coppers, such        as Cu-b, have an RRR lower than 8 and are avoided for        applications in which thermal conductivity is important.

Furthermore, the heat conducting wires of the braids are advantageouslyelectron beam welded to the mounting plates so that material continuityis guaranteed and the conduction of heat is feasible with the minimumtemperature drop (ΔT) between the cold end 121 of each cryorefrigeratorand the common cold end.

The above cryorefrigeration device has been described with N existingindividual cryorefrigerators. However, the invention also relates to acryorefrigeration device (not shown) comprising a single or “integrated”cryorefrigerator, incorporating all the preceding functions, for exampleusing N independent refrigeration circuits each comprising a cold endconnected to a common cold end.

The cryorefrigerators are very advantageously identical, that is to say,having the same thermal response in their operating cycles, thisresponse being more or less symmetrical about a mean temperature.Typically, two identical cryorefrigerators are of the same brand and thesame model.

The principle according to the invention of damping the thermaloscillations by phase-shift operation of the cryorefrigerators is veryeasily implemented when the cryorefrigerators have the same thermalresponse in their operating cycles, and when this response is more orless symmetrical about a mean temperature. The temperature generated atthe cold end of each of these machines is alternatively higher and lowerthan the mean temperature (thermal oscillations). If two identicalmachines are synchronized in phase opposition, one will have a highertemperature than the mean at the same time that the other has a lowertemperature.

For theoretical cyclic temperature response curves that are perfectlysymmetrical about the constant mean temperature (examples: sinusoidalresponse or rectangular or triangular shapes, etc.) the compensationwould be perfect and the resulting temperature would be constant andequal to the mean temperature. In practice, real response curves areneither sinusoidal nor even perfectly symmetrical about the meantemperature. The compensation is therefore not perfect. By usingidentical cryorefrigerators, the compensation tends more easily towardperfection. The settings of the control electronics serve to synchronizethe cryorefrigerators as well as possible, but do not serve to eliminatethe asymmetries. Hence the compensation is much more effective whenusing identical cryorefrigerators.

In practice, an attempt is made to approach symmetry most closely(allowing good compensation), by preferably selecting identicalcryorefrigerators, by controlling them to operate with exactly the samefrequency and with a phase difference (the phase being calculatedaccording to the number of cryorefrigerators used). If identical pulsetube refrigerators with two heat exchange stages are used (as shown inFIG. 6), the first stages of the cryorefrigerators are regulated atexactly the same temperature in order to make their operation, and hencetheir temperature response, as symmetrical as possible.

The implementation of the cryorefrigeration device according to thedevice is described below, with reference to FIGS. 3 to 5.

A cryorefrigeration device according to the invention, and describedabove, comprises N periodically operating cryorefrigerators 100 (or oneintegrated cryorefrigerator), where N is an integer equal to or greaterthan 2, each provided with a cold end 121 connected to a common cold end300. These N cryorefrigerators 100, or the integrated cryorefrigerator,are associated, according to the invention, with a control deviceprovided with a phase-shift means suitable for actuating a phase-shiftedoperation of the cryorefrigerators 100, or of the N refrigerationcircuits of the integrated cryorefrigerator, with regard to one another.

The result of an actuation of two cryorefrigerators in a phase-shiftedmanner of A with regard to B is shown in FIG. 3.

Tests were performed with two PT415 pulse tube refrigerators A and B(manufactured by CRYOMECH) connected by a mechanical uncoupling meansaccording to the invention (FIG. 2). A CERNOX type AA temperature sensor(manufacture by LAKESHORE) mounted at the center of the common cold endand connected to a rapid data acquisition system serves to observe thetemperature fluctuations.

Each cryorefrigerator is normally associated with a compressor and anelectronic module for control of the drive motor and its rotary valve.Each of these modules incorporates an oscillator and a power circuitwhich controls the motor.

For a preferred implementation of the invention, these modules weredisconnected and replaced by simple power modules without oscillator(MDP model MFM1CSZ34N7). The oscillators were replaced by adouble-output signal generator (TEKTRONIX model AGF3102). The generatoroutputs can be synchronized in normal operation or separated during theadjustment phase.

As shown in FIG. 3, the temperature behavior of each of the twocryorefrigerators A (dotted line) and B (dashed line) is not perfectlysinusoidal, but it is periodic. The mean temperature obtained in thiscase is 10.6 K. The temperature of the cold end of each of thecryorefrigerators A and B varies periodically between −800 mK and +800mK. When the two machines operate substantially in phase opposition, thetemperature variations are partly offset: the temperature variations ofthe common cold end (solid line) are lower than 100 mK in absolutevalue. In FIG. 3, the phase difference Δφ between the twocryorefrigerators is 177°, or about 2π/2 (N=2 in this embodiment). Ifthe cryorefrigeration device had comprised 3, 4 or N pulse tubes, thephase difference between each of the tubes would have been about 2π/3,2π/4, or 2π/N.

During normal operation of the device according to the inventionsubstantially in phase opposition (N=2) or in phase-shifted mode (Ngreater than 2), a misadjustment of the device may occur, so that thephase difference no longer serves to minimize the temperature variationsof the common cold end, or to maintain these temperature variationsbelow a threshold value. A step of adjusting the phase differencebetween each of the N cryorefrigerators is then necessary. This step isdescribed below, with reference to FIGS. 4 and 5.

Cryorefrigerators of the pulse tube type or Gifford-MacMahon type areperiodic machines in which the cycle is cadenced by a distributor drivenby a motor.

The synchronization of several machines, in order to smooth thetemperature fluctuations, requires identical speeds for thedistributors, but with an appropriate phase difference.

In an integrated system which comprises a plurality of refrigerationcircuits in the same device, it is possible to couple the distributorstogether mechanically, or even to produce a single distributor which, byconstruction, provides the necessary phase difference between therefrigeration circuits.

The implementation method according to the invention is described belowin relation to a device comprising N individual cryorefrigerators.However, this method is perfectly applicable to an integratedcryorefrigerator, comprising N refrigeration circuits.

To synchronize N identical independent cryorefrigerators, the inventionproposes making an “electrical tree” between the drive motors of thedistributors to obtain identical speeds (see FIG. 4). The invention alsoproposes allowing a temporary phase difference between the movements ofthese distributors during the adjustment phase, which consists inseeking the minimum of the temperature fluctuations or in reducing thetemperature fluctuations below a threshold value.

In pulse tube or Gifford-MacMahon cryorefrigerators, the distributor isdriven by a synchronous stepping motor. FIG. 4 shows the principle of acontrol system incorporating a system of automatic adjustment in phaseopposition for two cryorefrigerators. This system can obviously begeneralized to N cryorefrigerators.

The device measures the central temperature of the common end (“thermalcoupler”) 300 using a thermometer 310 and extracts the fluctuations ofthis temperature about the operating frequency by passband filtering andamplitude detection.

In the automatic position, the device makes a phase adjustment to adjustto the minimum amplitude of the fluctuations by adjusting thephase-shift means by increasing or decreasing increments according tothe sign of the derivative of the amplitude signal. When the adjustmentis completed, the control loop can be inhibited by switching to “locked”position. In the “manual control” position, the phase shift can beadjusted gradually and virtually continuously by an operator.

Thus, in general, the adjusting step comprises the following steps:

-   a1) operating the N cryorefrigerators simultaneously, in any    phase-shift manner;    -   b1) measuring the temperature variations of the common cold end        with the temperature sensor and calculating the mean temperature        of the common cold end;    -   c1) actuating a phase-shift operation between each        cryorefrigerator until the temperature variations of the common        cold end during the operation of the cryorefrigeration device        are lower than a threshold value in absolute value (preferably        less than 2% in absolute value of the mean temperature of the        common cold end, advantageously less than 1% in absolute value        of the mean temperature of the common cold end);    -   d1) setting the operating phase of each of the N        cryorefrigerators.

Obviously, the phase of all the N cryorefrigerators is not varied. Areference cryorefrigerator is arbitrarily selected among the Ncryorefrigerators, and the phase of the other N-1 cryorefrigerators isvaried so that all the N cryorefrigerators are offset with regard to oneanother, substantially by 2π/N.

According to a preferred embodiment, the operation of each of the Ncryorefrigerators is initially phase-shifted by 2π/N between them (stepa2) and not randomly phase-shifted. During a step c2), the phase of N-1cryorefrigerators is adjusted about their initial phase of step a2)until the temperature variations of the common cold end during theoperation of the cryorefrigerators device are lower than the thresholdvalue in absolute value.

FIG. 5 shows a recording of the thermal oscillations recorded in thecentral portion of the common cold end when the phase-shift between twopulse tube refrigerators is varied continuously, and for two meantemperature levels of the common cold end: 5 K (solid line) and 11 K(dotted line).

These measurements were taken with two PT415 pulse tube refrigerators Aand B (CRYOMECH) connected by a mechanical uncoupling means according tothe invention (FIG. 2). A temperature sensor mounted at the center ofthe common cold end is connected to a data acquisition system whichserves to observe the temperature fluctuations on a recorder.

FIG. 5 shows that the higher the mean temperature of the cold end, thewider the temperature variations may be. Thus, for a mean temperature of11 K, the temperature variations may reach almost 500 mK. For a meantemperature of 5 K, the temperature variations may reach nearly 200 mK.

By changing the frequency of one of the tubes with regard to the other(or more generally of N-1 tubes with regard to the final tube), thephase of the first tube is caused to slide continuously with regard tothe other: for a frequency difference of 0.6% between the two tubes, thephase difference reaches 180° (2π/2) in about 60 s.

In the graph in FIG. 5, the maximum variation in the mean temperature ofthe common cold end is obtained when the tubes A and B are in phase(substantially at 3 and a half seconds). The minimum variation in meantemperature of the common cold end is obtained when the tubes A and Bare in phase opposition (substantially at half a second and at 7seconds).

Once the adjustment procedure is completed, the outputs of the generatorare synchronized to the set the operating phase of each of the Ncryorefrigerators.

A second embodiment (not shown) consists of an electronic circuit boardcombining the functions described in the diagram in FIG. 4, and asimplified man-machine interface for controlling the adjustments andmonitoring the level of fluctuations.

The device described above is based on the use of two commercial pulsetube refrigerators, as they exist in catalogues. As shown in FIG. 6, itis similarly possible to combine 3, 4 or N cryorefrigerators 100, byconnecting their cold ends by a common cold end, and by phase-shiftingtheir operation by 2π/N. The larger the number N of cryorefrigerators,the smoother the temperature response of the common cold end, that is tosay, having small thermal oscillations.

If pulse tube refrigerators with two heat exchange stages 102-103 areused (as shown in FIG. 6), it may be advantageous to connect the firststage 102 of each tube 100 together thermally by a flexible heatconducting braid 104, thereby serving to create a first “common” stageusable for cooling thermal screens, for example. The problem of thetemperature drop ΔT lost is much less important than for the common coldend of the second stage 103.

In this embodiment, the phase difference of the variouscryorefrigerators is obtained by a common rotary valve 500, distributingthe high pressure and the low pressure to the various cryorefrigerators,through openings made therein. The rotary valve 500 is actuated by amotor 501 controlled by a control device 600 incorporated in thecompressor.

1. A cryorefrigeration device comprising N periodically operatingcryorefrigerators, where N is an integer equal to or greater than 2,each provided with a cold end connected to a common cold end, thecryorefrigerators being associated with a control device provided with aphase-shift means suitable for actuating phase-shifted operation of thecryorefrigerators with regard to one another, characterized in that thecold end of each cryorefrigerator is connected to the common cold endvia a heat conducting mechanical uncoupling means.
 2. The device asclaimed in claim 1, in which the phase-shift means is suitable foractuating a phase-shifted operation with a phase difference of 2π/N, towithin 5 degrees, between each cryorefrigerator.
 3. The device asclaimed in claim 1, in which the cryorefrigerators are identical.
 4. Thedevice as claimed in claim 1, in which the cryorefrigerators are pulsetube refrigerators or Gifford-MacMahon cryorefrigerators.
 5. The deviceas claimed in claim 1, in which the heat conducting mechanicaluncoupling means comprises a plurality of heat conducting wire braidsfixed between two mounting plates in thermal contact with the cold endof a cryorefrigerator and the common cold end, respectively.
 6. Thedevice as claimed in claim 5, in which the heat conducting wires of thebraids are made from Cu/a1 copper and have a diameter between 0.03 mmand 0.1 mm, preferably equal to 0.05 mm, and the mounting plates have aresidual resistivity ratio of at least
 50. 7. The device as claimed inclaim 5, in which the mounting plates and/or the common cold end aremade from Cu/a1 or Cu/c1 copper.
 8. The device as claimed in claim 5, inwhich the heat conducting wires of the braids are welded to the mountingplates by electron beam welding so as to ensure material continuity. 9.The device as claimed in claim 1, in which the common cold end comprisesa temperature sensor connected to the control device.
 10. A method forusing a cryorefrigeration device as claimed in claim 1, comprising astep of actuating the N cryorefrigerators in a phase-shifted manner withregard to one another, characterized in that it comprises a step ofadjusting the phase difference between each of the N cryorefrigerators,this adjustment step comprising the following steps: a1) operating the Ncryorefrigerators simultaneously, in any phase-shifted manner; b1)measuring the temperature variations of the common cold end with thetemperature sensor and calculating the mean temperature of the commoncold end; c1) actuating a phase-shifted operation between eachcryorefrigerator until the temperature variations of the common cold endduring the operation of the cryorefrigeration device are lower than athreshold value, preferably less than 2% in absolute value of the meantemperature of the common cold end, advantageously less than 1% inabsolute value of the mean temperature of the common cold end; d1)setting the operating phase of each of the N cryorefrigerators.
 11. Themethod for using a cryorefrigeration device as claimed in claim 1,comprising a step of actuating the N cryorefrigerators in aphase-shifted manner with regard to one another, characterized in thatit comprises a step of adjusting the phase difference between each ofthe N cryorefrigerators, this adjustment step comprising the followingsteps: a2) operating the N cryorefrigerators simultaneously, with aphase difference of 2π/N between each cryorefrigerator; b2) measuringthe temperature variations of the common cold end with the temperaturesensor and calculating the mean temperature of the common cold end; c2)varying the phase of N-1 cryorefrigerators about their initial phase instep a) until the temperature variations of the common cold end duringthe operation of the cryorefrigeration device are lower than a thresholdvalue, preferably less than 2% in absolute value of the mean temperatureof the common cold end, advantageously less than 1% in absolute value ofthe mean temperature of the common cold end; d2) setting the operatingphase of each of the N cryorefrigerators.